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PD-L1 is a protein that allows some cancer cells to escape degradation by the immune system. PD-L1 on the cancer cell surface interacts with a protein called PD-1 on T cells. This coupling (known as an immune checkpoint) instructs the T cell to leave the tumor cell alone as a result of T-cell anergy Strong PD-L1 expression on the tumor cells is noted here and hence checkpoint inhibitors such as nivolumab are often used in targeted therapy.